1. Field of the invention
This invention relates to X-ray analysis apparatus, particularly but not exclusively apparatus for computer tomography using X-rays, and also to X-ray targets and collimators for X-rays, for use in such X-ray apparatus. The principles of computer tomography using X-rays are well known, and need not be described here. "Computer tomography" will be abbreviated to CT, in this description.
2. Description of the prior art
Recently CT systems have been developed for the testing of products for industrial uses. Since such products, e.g. metal products, are generally high in density, testing radiations such as X-rays, gamma-rays or light rays have too low transmittance when their energy is low. Therefore, there is a need to raise the energy level of the testing radiation. Moreover, the spatial resolution sought is as high as 0.2 mm. However, when the testing radiation has high energy, problems discussed below arise, and especially high resolution is difficult of realization. These demands are high particularly for an X-ray CT system. It is now proposed to employ a charged particle beam, such as an electron beam.
As described in "Recent Medical Diagnostic Systems" (1988), pages 86-89, the typical X-ray generator of a CT system is one in which a high voltage is applied across a cathode and an anode which are arranged in the interior of a high vacuum tube, and electrodes accelerating from the cathode impinge against the X-ray generating target of the anode (in this description simply called "target") in order to generate X-rays.
U.S. Pat. No. 4,607,380 shows a rotating anode, which has customarily been used in order to reduce damage of the anode by the impinging electron beam. The beam is focussed, passes through an aperture plate and is then focussed again by focussing magnets to converge it onto the rotating anode. The anode is in a vacuum chamber connected through the aperture to the vacuum chamber containing the electron source. The X-ray are emitted from the surface of the anode which is bombarded by the electron beam. This apparatus is described as useful for the industrial inspection of objects, such as thick metal parts and achieves a small focal spot with high power output, at the anode.
U.S. Pat. No. 4,573,185 shows a rotating anode, without mentioning any particular use of it. A focal track of tungsten is arranged on a substrate and has a width narrower than that of the electron beam from the cathode, so that misalignment between the beam and the track will not affect the effective focal spot size or location.
In the Journal of Nuclear Science and Technology (Japan) 26 (1989), pages 826-832, there is described a prototype high energy X-ray CT system for use in imaging dense and large objects. A linear accelerator is employed to accelerate the electron beam and to produce X-ray photons with a maximum energy of 12 MeV. When using an X-ray source of such high energy, however, there is a problem that even a rotating anode cannot be sufficiently cooled in order to avoid damage to the surface bombarded by the energy beam.
First, problems of an X-ray generator using an electron beam will be discussed. The high energy of the X-rays signifies that the energy of the electrons is also high. The first problem of the X-ray generator is that the high energy of the electrons enlarges the size of the X-ray generation region and thereby worsens the spatial resolution. When the accelerated electrons are of at most about 200 keV, the energy levels of the generated electrons are comparatively uniform. Therefore, the electrons defocus little on the target, and the X-ray generation region of 0.2 mm has been realizable without using any special convergence means. However, when the energy of the electrons becomes 1 MeV or above, the dispersion of the energy of the electrons widens, and also the defocussing of the electrons on the target increases, so that the X-ray generation region is enlarged.
A second problem is that the vacuum performance of the X-ray generator degrades, so the necessary spatial resolution fails to be achieved. In the prior art, the entire target is made up of an X-ray generating target element of insufficient heat transfer coefficient. Besides, in the prior art, the target is installed in the same vacuum as that of the device for generating the electrons. Consequently, the large amounts of gases produced from the bombarded part (namely, X-ray generating region) of the target, when the electrons have high energy, degrades the vacuum performance of the X-ray generator. When the vacuum is degraded, the generated electrons are not sufficiently accelerated on account of the gases, and the necessary energy cannot be reached. As a result, it becomes impossible to obtain X-rays having sufficient energy and to attain the necessary spatial resolution. Moreover, when the vacuum is degraded, the electrons undergo scattering due to the gases, and the X-ray generating region enlarges. In consequence of this also, spatial resolution deteriorates.
A third problem is that the efficiency of maintenance worsens. When bombarded with electrons of high energy, the target is damaged more, so that the frequency of replacement of the target rises. Nevertheless, replacement is troublesome because the target is installed within the vacuum region.
A further area where problems arise in a CT system particularly one for imaging of industrial objects, is the collimator in front of the detector. Resolution can be increased by making the slit widths thinner, provided that the intensity of the X-rays penetrating the scanned object is sufficient. In JP-A-57-1329, there is shown a collimator for a CT system having an adjustable slit width. Slits are provided in three spaced apart, superposed plates, and the middle plate is shifted in order to adjust the effective slit width. A problem which arises is that the high energy X-rays required for industrial CT may penetrate the plate, as well as the slit, leading to loss of resolution. In the field of medical CT, JP-A-57-180944 discloses a pre-collimator for the X-rays, i.e. a collimator in the path of the beam before the beam encounters the patient, in which a plurality of sets of slits are provided in a disk, and the desired set is selected by rotating the disk.
Problems arising with the collimator are as follows. In the first place, enhancement in resolution is problematic. The resolution is enhanced as the distance between the radiation source and the test piece is shortened and the distance between the test piece and the collimator (detector) can be lengthened, so far as possible in accordance with the size of the test piece. The reason is that, since the testing radiation is generally emitted radially from the radiation source, the width of the test piece to be detected by one slit of the collimator can become narrower to enhance the resolution as the former distance is shorter and the latter distance is longer. In order that the testing radiation emitted from the radiation source may pass without colliding against the collimator, also the collimator slits need to maintain a radial positional relationship with the radiation source, despite variations in the above positional relationships. This point is not taken into consideration in the prior art.
A second problem concerns space. The shielding capability of the collimator is determined by the length of the slit, namely the thickness of the collimator. The construction of JP-A-57-1329 mentioned above has the problem that the respective slit groups require lengths necessary for shielding, so a large collimator space is necessitated. When the energy of the testing radiation is high, this drawback becomes especially serious. On the other hand, in the construction of JP-A-57-180944, the slit groups are radially arranged, so that the use of space in inferior. Particularly in the case where the energy of the testing radiation is high, there is the problem that the whole system becomes heavy in weight, incurring also the enlargement of the size of a collimator driving section.